Bullet or projectile speed measuring apparatus may use a measuring path, traversed in flight by the projectile, which has two light gates disposed at a predetermined distance from one another along it. Each light gate has a light responsive element, which is shielded by a screen or shield having apertures from the projectile path. Such apparatus also includes an electronic signal evaluation circuit, which processes signals emitted in succession by the two light responsive elements in accordance with the darkening caused by the projectile traveling above them and delivers these signals to a display device.
An apparatus of this kind known in practice uses two separate light gates, each of which has a directionally sensitive light responsive element accommodated in its own housing having a light-admitting aperture disposed crosswise to the projectile path. The light responsive elements are illuminated through the apertures, either by daylight or by the light of lamps disposed above them. The brief darkening of the aperture that is caused by the flight of a projectile over it causes the light responsive element to emit a corresponding signal. Time counters of the associated signal evaluation circuit measure the period of time that has elapsed between the appearances of signals from the two light responsive elements, and the projectile speed can be calculated from the length of this period, taken together with the distance between the two light gates.
The two light gates, each with its own housing, are disposed one after the other on a rail-like base plate, with the attendant danger that a light gate, particularly the one located farther from the muzzle of the weapon, may be struck by the projectile. The result is not only the possible destruction of the apparatus; persons may also be endangered by the ricocheting projectile or by brokenoff fragments. Aside from this, the use of a plurality of separate parts is associated with the danger of unreliable measurements, because the distance between the two light gates is part of the calculation of the projectile speed and must accordingly be established very precisely. The projectile speed must be calculated on the basis of the time interval and the distance between light gates in a separate operation.
The danger of mistakenly striking part of the measuring apparatus with the projectile is overcome, in another apparatus known in practice by providing that the apparatus, which again has two light gates spaced apart from one another, be mounted directly upon the barrel of the weapon. In so doing, however, it is unavoidable that the vibrations produced in the barrel when the weapon is fired will be transmitted to the light gates, which may impair their function and accuracy of measurement. Also, the length and weight of the apparatus may not exceed a certain, relatively low minimum amount, because otherwise the weapon becomes overly difficult to handle. Frequently the alterations that must be made to the barrel of the weapon in order to mount the apparatus are also undesirable, and in fact a certain change in firing conditions caused by mounting such an apparatus on the barrel of the weapon cannot be avoided.
Still other known measuring apparatuses therefore dispense entirely with the use of light gates, and instead use electrically conductive foil strips disposed spaced apart from one another, which are penetrated in succession by the projectile in flight. These apparatuses are, however, limited in their measuring accuracy and are relatively susceptible to disturbance factors.